Electromagnetic systems for surveying soils and the like generally involve the measurement of one or more electric or magnetic field components induced in the earth subsurface by changes in a primary field produced from variations in a current source. The primary field spreads out in space both above and below the ground and its change induces currents in subsurface conductors in accordance with the laws of electromagnetic induction. These currents give rise to secondary magnetic fields, which distort the primary field. In general, the resultant secondary field, the change of which may be picked up by a receiving coil, will differ from the primary field in intensity, and direction and reveal the presence of a conductor.
Electromagnetic methods are sensitive to variations in electrical properties of subsurface materials and can map out regions with enhanced conductivity due to the presence of fluids, metals, or other variations. Electromagnetic induction instruments induce currents in conductors within the subsurface without having to make direct contact with the ground.
Electromagnetic methods can be utilized to:
Map conductive soil and groundwater contamination.
Map soil moisture and salinity in agricultural areas.
Characterize subsurface hydrogeology (map buried channel deposits, groundwater exploration, locate conductive fault and fracture zones).
Characterize Geological Structure.
A typical electromagnetic method for mapping a subsurface geology includes placing a loop of wire (typically 10-300 meters square) on the ground surface. A steady current is passed through the loop and the transmitter emits a primary electromagnetic field for surveying ground heterogeneity beneath where the survey system is located. A receiver coil in the centre of or offset from the transmitter loop records the resultant electromagnetic field for processing and interpretation, with the resultant electromagnetic field corresponding to the interaction of the primary electromagnetic field with the underlying substrate.
The recorded or resultant electromagnetic field generally reflects a combination of the primary electromagnetic field emitted by the transmitter loop, as well as a secondary electromagnetic field emanating from the underlying substrate. Typically the secondary electromagnetic field emanating from the substrate is generally much smaller in amplitude than the primary electromagnetic field. A drawback of existing methods and systems is that often the primary electromagnetic field can overwhelm the receiver and interfere with its ability to sense the secondary electromagnetic field hence a reduced ability of the receiver to sense the secondary electromagnetic field.
It is known that the effect of interference from the primary electromagnetic field can be reduced by maintaining large separation between the transmitter and the receiver. One method is adopted where wire loops are manually laid on the ground and transmitter loop and receiver loop manually separated by a distance. However, with increased separation also comes further problems including one of interpretation of the resultant data wherein shallow resistivity variation with depth cannot be uniquely interpreted, and a second problem whereby if a large area is required to be surveyed, manual separation of loops is both inconvenient and tedious.
Some attempts have been made to improve efficiency by towing a transmitter and receiver loops behind land or water based vehicles. For example, a recent system has included a transmitter adapted for towing by a tow vehicle and a sled type structure supporting a receiver loop separated from the transmitter which is drawn in proximity to the ground over the surface behind the transmitter loop. In this apparatus, the strength of the primary electromagnetic field drops off steeply with distance from the outside of the transmitter. The receiver is therefore more readily able to detect the secondary electromagnetic field.
While this approach improves the rate of measurement in terms of area of mapping per time, and separates the transmitter and receiver loops sufficiently to reduce interference of the secondary signal, the apparatus is impractical because it is akin to a road train to the extent that it has very poor towing and turning circle ability, and reduced suitability for off-road surveys, and the strength of the secondary signal may be distorted by changes in ground heterogeneity between the transmitter and receiver in an un-interpretable manner as the receiver loop traverses the ground.
Some towed electromagnetic carts have been created where the transmitter loop support has been heavy and cumbersome. The transmitter loop must be large both for increasing depth of penetration and for increasing range of depth of investigation by minimizing the number of turns, that raise internal inductance, needed to transmit a magnetic moment of equivalent strength. A cart design is needed that is robust when towed through farmland but also easy to man-handle.
Other attempts have been made to increase the efficiency of subsurface mapping by providing airborne systems, however these systems are expensive, large and have large sampling footprints not suited to shallow and detailed exploration or to exploration of substrate in areas littered with numerous metallic features.
There remains therefore an ongoing need to provide an improved electromagnetic survey apparatus for transmitting and receiving electromagnetic fields to determine the features of the underlying substrate in an efficient and practical way. It would be particularly advantageous if the electromagnetic survey system allowed for reduced interference of the primary electromagnetic field with the receiver while providing a convenient and easy means for traversing and therefore mapping a large area.
It is an object of the present invention to substantially overcome or at least ameliorate one or more of the disadvantages of the prior art.